Batteries with permanently wet-able fine fiber separators

ABSTRACT

Alkaline batteries are disclosed that advantageously include separators comprising at least one porous layer of fine fibers having a diameter of between about 50 nm and about 3000 nm that provide improved combinations of reduced thickness, dendritic barrier against short-circuiting and low ionic resistance as compared with known battery separators. The fine fibers show improved wet-ability in the alkaline electrolytes.

FIELD OF THE INVENTION

The present invention relates to batteries with permanently wet-ablefine fiber separators with surface active agents.

BACKGROUND OF THE INVENTION

Polymer fibers have been widely used in the nonwovens industry in themanufacture of nonwoven webs, fabrics, and composites. Olefin polymers,such as polyethylene, polypropylene, polybutene, polypentene, andcopolymers of ethylene or propylene with other olefinic monomers, areknown for their hydrophobic properties. Thus, nonwoven webs ofpolyolefin fibers are frequently used in applications where theirhydrophobic properties are advantageous. For example, polyolefinnonwovens are often used in diapers, other hygiene products and medicalapplications where it is desired to keep moisture away from a wearer'sskin.

However, there are numerous other nonwoven fabric applications where thehydrophobic nature of polyolefin fibers is not required and wherehydrophilic properties are desired. Even a fiber made of a polymer suchas a polyamide or polyester may not have the required hydrophilicity forcertain applications. If a nonwoven fabric formed of polymer fibers isto be used, the fibers must be treated in some way to alter the normallyhydrophobic properties of the fibers to impart hydrophilic properties.One well-known practice involves the topical application ofcompositions, such as surfactants, to render the fabric morehydrophilic. However, topical chemical applications are not entirelysatisfactory for some applications, since they are not durable. Thehydrophilic property is lost after washing or after extended use in abattery. The extra processing steps required for topical chemicaltreatments or other fiber surface modification treatments alsoundesirably increase the cost of the fabric. The few processes known torender the polymers wet-able are environmentally unfriendly, relativelyslow and have limited durability.

For improving wet-ability it is known in the industry that certainsurfactants, such as TRITON X-100 from Rohm and Haas, can be applied asan aqueous solution or suspension to the surface of hydrophobic fibers,filaments or nonwoven fabrics with the resulting effect of rendering thefibers, filaments or fabrics wet-able, although not absorbent. Thesetopical treatments can be applied by any means familiar to one skilledin the art, such as foaming spraying, dip-and squeeze or gravure roll.In almost every case, some sort of heating step is required to removeresidual water or solvent used to prepare the surfactant solution orsuspension. This step adds significantly to the manufacturing costs andcomplexity. Further, thermoplastics are altered by exposure to heat andcareful monitoring of the heating process is required to ensure thatfabric properties are not adversely affected. Also, since surfactantsare not strongly chemically bonded to the fiber or filament surfaces,such topical treatments are not durable. They tend to wash off duringrepeated fluid exposures or rub off during use.

In an effort to correct this deficiency, corona discharge treatmentshave been used to alter the electrochemical potential of the surfaces offibers or filaments. The effect is to render surfaces more reactive withthe result that hydrophobic surfaces become more wet-able. However,these electrical potential changes are also not permanent, beingparticularly subject to environmental effects, such as storage in moistenvironments.

An additional alternative is the use of surface chemical treatmentswhere the surfactants are covalently bonded to the polymer.

Another approach is the incorporation of chemical agents in thethermoplastic polymer before it is melt extruded into fibers, filamentsor nonwoven fabrics, rendering the fibers themselves hydrophilic.Agents, such as siloxanes, have been proposed for this purpose. Here,the object is to impart a durable change in the wet-ability of thefibers or filaments. The performance model theory states that the meltadditives become dispersed in the molten polymer and are bound in thematrix when the polymer cools during fiber or filament quenching. Overtime, due to the effects of further processing, the additive rises tothe surface of the fibers or filaments, a phenomenon called blooming,imparting durable wet-abilty. Published PCT Patent SpecificationWO99/00447 discloses a product and process for making wet-able spunbondand melt blown fibers prepared from an olefin polymer, polyester orpolyamide including a wetting agent consisting essentially of amonoglyceride or a combination of a monoglyceride and a mixed glyceridewith the monoglyceride amounting to at least 85% by weight in the caseof the combination.

However, the use of hydrophilic melt additives can add significantly tothe cost of the nonwoven webs. Also, the addition of a hydrophilic meltadditive to the polymer can alter the properties of the fibers orfilaments, resulting in unacceptable changes to important physical oraesthetic properties of the nonwoven web, such as strength, softness orhand, for example.

In alkaline batteries, a separator is used between a positive electrodeand a negative electrode to keep them separated and to prevent a shortcircuit therebetween, and further, to hold an electrolyte thereon andenable a smooth electromotive reaction.

Battery separators for alkaline batteries are conventionally eitherthick, multi-layered nonwovens having large pores that have good (low)ionic resistance but relatively poor barrier to growing dendrites (alsoreferred to herein as “dendritic barrier”), or multi-layered nonwovenswith microporous membranes thereon having very small pores that havegood dendritic barrier but very high ionic resistance.

The space allotted for the battery has been becoming smaller inelectronic equipment, due to the need for miniaturization andweight-saving. Nevertheless, the performance requirement for such asmaller battery is the same as or higher than that for a conventionalbattery, and therefore, it is necessary to enhance the capacity of thebattery, and to increase the amounts of active materials in theelectrodes. Thus, it would be advantageous if the volume allotted in thebattery for the separator could be reduced, and the separator madethinner. However, if a conventional separator is simply made thinner,its capacity for holding electrolyte (i.e., the electrolyte-holdingcapacity) is reduced. Thinner nonwovens with large fibers results inlarge effective pore size of the separator and poor barrier properties.In addition, in these thinner nonwovens, the uniformity of the fiberdistribution may be reduced, further increasing the effective pore size.

U.S. Pat. No. 7,112,389 is describes the use of nanowebs as separatorsin batteries. The use of nanowebs yields superior performance by givinga better balance between the ionic resistance and barrier properties.Because the fiber size is drastically reduced, as compared toconventional battery separator materials, a very small pore size can beachieved with very thin separator materials. The nanowebs are coatedwith surfactants to improve the wet-ability, wicking, and electrolyteabsorption in 35% KOH.

There remains a need for thin separators with permanent wet-ability foruse in batteries which also exhibit good wicking and electrolyteabsorption properties.

SUMMARY OF THE INVENTION

One embodiment of the present invention is directed to an alkalinebattery having a separator comprising a porous fine fiber layer ofwet-able polymeric fibers having a mean diameter in the range from about50 nm to about 3000 nm, wherein the porous fine fiber layer permanentlywets with strong alkaline electrolytes.

DETAILED DESCRIPTION OF THE INVENTION

It would be desirable to have alkaline batteries with thin separatorshaving an improved balance of dendritic barrier and ionic resistance, aswell as permanent wet-ability in 35% KOH electrolyte, which also havegood wicking and electrolyte absorption properties.

The alkaline batteries of the present invention include batteryseparators having an improved combination of reduced thickness, reducedionic resistance and good dendritic barrier properties, providing a highresistance to short-circuiting. The separators useful in the batteriesof the invention have a high capacity to absorb electrolyte whilemaintaining excellent structural integrity and chemical and dimensionalstability in use, such that the separators do not lose their dendriticbarrier properties even when saturated with electrolyte solution. Thereduction in thickness enables the manufacture of batteries havingincreased capacity. The separators useful in the batteries of theinvention have low ionic resistance, therefore ions flow easily betweenthe anode and the cathode. The separators wet instantly with alkalineelectrolytes and stay wet during extended use in the battery.

The specific polymer compositions of the present invention, when spuninto fine fiber layers, have superior wetting properties in strongalkaline solutions and also exhibit good wicking and electrolyteabsorption properties with thinner separators.

By “wet-able fiber” is meant that the polymeric fibers of the batteryseparator of the invention comprise surfactant molecules embedded withinthe polymer and extending to the fiber surfaces.

In one embodiment of the invention an effective amount of surfactant isadded to the fiber spinning solution to form polymeric fibers comprisinga composition of polymer and surfactant which is wet-able. An “effectiveamount” of surfactant is to be understood to mean at least an amountthat produces wet-ability in the electrolyte of choice. This procedureresults in a good distribution and entrapment of the surfactantmolecules in the fibers and on the fiber surface. In turn, thisminimizes both the risk of surfactant molecules being stripped from thesurface of the fibers during handling of the material and the risk ofsurfactant molecules leaching into the electrolyte in the end use.

In one embodiment of the invention, an effective amount of surfactant isadded to the fiber spinning solution to form polymeric fibers comprisinga composition of polymer and surfactant which makes the fibers, whichare inherently hydrophobic, hydrophilic. The fibers can then be furthercoated with water-based surfactant solutions after spinning, instead ofhaving to be coated with a non-aqueous surfactant solution. The polymercan be coated with surfactant from aqueous solutions and the separatorwill be wet-able in the electrolyte of choice when it is dry.

Suitable surfactants of the invention are preferably nonionicsurfactants, such as alkylated polyether surfactant (e.g. Tergitol orTriton from Dow Chemical) or siloxyl polyether surfactant (e.g. Silwetfrom GE), but not limited to them. The surfactant amount can vary from0.4 wt % to 20 wt % (weight % relative to the polymer), preferablybetween 1 wt % and 5 wt %.

One embodiment of the invention relates to an alkaline battery. Thebattery can be an alkaline primary battery, e.g., Zinc-Manganese Oxideor Zn—MnO₂ battery in which the anode is zinc and the cathode ismanganese oxide (MnO₂), or Zinc-Air battery in which the anode is zincand the cathode is air, or it can be an alkaline secondary battery,e.g., a Nickel Cadmium battery in which the anode is cadmium and thecathode is Nickel oxy-hydroxide (NiOOH), Nickel Zinc or Ni—Zn battery inwhich the anode is zinc and the cathode is NiOOH, Nickel Metal Hydride(NiMH) battery in which the anode is metal hydride (e.g. LaNi₅) and thecathode is NiOOH or Nickel-Hydrogen or NiH₂ battery in which the anodeis hydrogen (H₂) and the cathode is NiOOH. Other types of alkalinebatteries include Zinc/Mercuric Oxide in which the anode is zinc, andthe cathode is mercury oxide (HgO), Cadmium/Mercuric Oxide in which theanode is cadmium and the cathode is mercury oxide, Zinc/Silver Oxide inwhich the anode is zinc and the cathode is silver oxide (AgO),Cadmium/Silver Oxide in which the anode is cadmium and the cathode issilver oxide. All of these battery types use 30-40% potassium hydroxideas the electrolyte.

The battery of the present invention includes a separator having atleast one porous layer of wet-able fine polymeric fibers having a meandiameter in the range of between about 50 nm and about 3000 nm, evenbetween about 50 nm and about 1000 nm, and even between about 50 nm andabout 500 nm. Fine fibers in these ranges provide a separator structurewith high surface area which results in good electrolyte absorption andretention due to increased electrolyte contact. The separator has a meanflow pore size of between about 0.01 μm and about 15 μm, even betweenabout 0.01 μm and about 5 μm, and even between about 0.01 μm and about 1μm. The separator has a porosity of between about 20% and about 90%,even between about 40% and about 70%. The high porosity of the separatoralso provides for good electrolyte absorption and retention in thebattery of the invention.

A separator useful in the battery of the invention has a thickness ofbetween about 0.1 mils (0.0025 mm) and about 12 mils (0.3 mm), evenbetween about 0.5 mils (0.0127 mm) and about 5 mils (0.127 mm). Theseparator is thick enough to prevent dendrite-induced shorting betweenpositive and negative electrode while allowing good flow of ions betweenthe cathode and the anode. The thin separators create more space for theelectrodes inside a cell and thus provide for improved performance andlife of the batteries of the invention.

The separator has a basis weight of between about 1 g/m² and about 90g/m², preferably between about 5 g/m² and about 30 g/m². If the basisweight of the separator is too high, i.e., above about 90 g/m², then theionic resistance may be too high. If the basis weight is too low, i.e.,below about 1 g/m², then the separator may not be able to reducedendrite shorting between the positive and negative electrode.

The separator has a Frazier air permeability of less than about 150cfm/ft² (46 m³/min/m²), even less than about 25 cfm/ft² (8 m³/min/m²),even less than about 5 cfm/ft² (1.5 m³/min/m²). In general, the higherthe Frazier air permeability, the lower the ionic resistance of theseparator, therefore a separator having a high Frazier air permeabilityis desirable.

The separator can comprise multiple porous fine fiber layers which maycomprise the same or different polymers. In addition, the multiplelayers may have differing characteristics selected form the listconsisting of thickness, basis weight, pore size, fiber size, porosity,air permeability, ionic resistance, and tensile strength.

Suitable polymers for use in the alkaline battery separator includealiphatic polyamide, semi-aromatic polyamide, polyvinyl alcohol,cellulose, polyethylene terephthalate, polypropylene terephthalate,polybutylene terephthalate, polysulfone, polyvinylidene fluoride,polyethylene, polypropylene, polymethyl pentene, polyphenylene sulfide,polyacetyl, polyacrylonitrile, polyurethane, aromatic polyamide andblends, mixtures and copolymers thereof. Polymers that are especiallysuitable for use in the alkaline battery separator include polyvinylalcohol, cellulose, aliphatic polyamide and polysulfone. In someembodiments of the invention, it may be preferable to crosslink thepolymeric fine fibers in order to maintain the porous structure andimprove the structural integrity of the separator in the electrolyte.For example, uncross linked polyvinyl alcohol separators can dissolve inwater and form a gel type structure having poor structural integrity instrong alkaline electrolytes. Certain polymers, e.g. polyvinyl alcohol(PVA), polyvinylidene fluoride, polyvinylidenefluoride-hexafluoropropylene, polyethylene oxide, polyacrylonitrile,polymethyl methacrylate, tend to swell or gel in the electrolytes, thusclosing the pores of the fibrous structure. In certain cases they willalso become soft or degrade in the electrolyte leading to poorstructural integrity. Depending on the polymer of the battery separator,various cross linking agents and cross linking conditions can be used.All the polymers mentioned above can be cross linked by known means,such as by chemical cross linking, electron beam cross linking or UVcross linking.

One process for making the fine fiber layer(s) of the separator for usein the battery of the invention is an electroblowing process asdisclosed in International Publication Number WO2003/080905 (U.S. Ser.No. 10/477,882), which is hereby incorporated by reference.Alternatively, the fine fiber layer(s) of the separator can be made by aconventional electrospinning process such as disclosed in U.S. PublishedPatent Application No. 2004/0060268 A1 (now U.S. Pat. No. 6,924,028).

In one embodiment of the invention, the battery separator comprises asingle fine fiber layer made by a single pass of a moving collectionmeans through the process, i.e., in a single pass of the movingcollection means under the spin pack. Alternatively, the batteryseparator can comprise multiple fine fiber layers, formed by multiplepasses under the spin pack. It will be appreciated that the fibrous webcan be formed by one or more spinning beams running simultaneously overthe same moving collection means. When the separator comprises multiplelayers, the multiple layers can be layers of the same polymeric finefibers, or can alternatively be layers of differing polymeric finefibers. The multiple layers can have differing characteristicsincluding, but not limited to, polymer, thickness, basis weight, poresize, fiber size, porosity, air permeability, ionic resistance andtensile strength.

The collected fine fiber layer(s) are advantageously bonded which hasbeen found to improve the tensile strength of the separator. A highlevel of tensile strength in the machine direction helps during cellwinding and also contributes to the good dendritic barrier of theseparator in use. Bonding may be accomplished by known methods,including but not limited to thermal calendering between heated smoothnip rolls, ultrasonic bonding, point bonding, and through gas bonding.Bonding increases the strength of the fine fiber layer(s) so that thelayer(s) may withstand the forces associated with being handled andbeing formed into a useful separator, and depending on the bondingmethod used, adjusts physical properties such as thickness, density, andthe size and shape of the pores.

Test Methods

Basis Weight was determined by ASTM D-3776, which is hereby incorporatedby reference and reported in g/m².

Porosity was calculated by dividing the basis weight of the sample ing/m² by the polymer density in g/cm³ and by the sample thickness inmicrometers, multiplying by 100 and subsequently subtracting from 100%,i.e., percent porosity=100-basis weight/(density×thickness)×100.

Fiber Diameter was determined as follows. Ten scanning electronmicroscope (SEM) images at 5,000× magnification were taken of each finefiber layer sample. The diameter of eleven (11) clearly distinguishablefine fibers were measured from the photographs and recorded. Defectswere not included (i.e., lumps of fine fibers, polymer drops,intersections of fine fibers). The average (mean) fiber diameter foreach sample was calculated.

Thickness was determined by ASTM D1777, which is hereby incorporated byreference, and is reported in mils and converted to micrometers.

Frazier Air Permeability is a measure of air permeability of porousmaterials and is reported in units of ft³/min/ft². It measures thevolume of air flow through a material at a differential pressure of 0.5inches (12.7 mm) of the water. An orifice is mounted in a vacuum systemto restrict flow of air through sample to a measurable amount. The sizeof the orifice depends on the porosity of the material. Frazierpermeability is measured in units of ft³/min/ft² using a Sherman W.Frazier Co. dual manometer with calibrated orifice.

Mean Flow Pore Size was measured according to ASTM Designation E1294-89, “Standard Test Method for Pore Size Characteristics of MembraneFilters Using Automated Liquid Porosimeter” which approximately measurespore size characteristics of membranes with a pore size diameter of 0.05μm to 300 μm by using automated bubble point method from ASTMDesignation F 316 using a capillary flow porosimeter (model numberCFP-34RTF8A-3-6-L4, Porous Materials, Inc. (PMI), Ithaca, N.Y.).Individual samples (8, 20 or 30 mm diameter) were wetted with lowsurface tension fluid (1,1,2,3,3,3-hexafluoropropene, or “Galwick,”having a surface tension of 16 dyne/cm). Each sample was placed in aholder, and a differential pressure of air was applied and the fluidremoved from the sample. The differential pressure at which wet flow isequal to one-half the dry flow (flow without wetting solvent) is used tocalculate the mean flow pore size using supplied software.

Wetting time (seconds) is measured by dispensing 1 μl of fluid (water or20% KOH solution) onto the sample surface and timing how long it takesto soak into the sample. The fluid is dispensed by an automatic syringethat delivers the same amount each time. The wetting time is reported inseconds.

Electrolyte Absorption is measured by soaking 10 cm×10 cm samples in 35%KOH for 10 minutes. The weight of the samples is measured before andafter soaking in 35% KOH and electrolyte absorption is calculated withthe formula:

${\% \mspace{14mu} {{Elec}.{Abs}.}} = {\frac{\left( {W_{f} - W_{i}} \right)}{W_{i}} \times 100}$

Where W_(f), and W_(i) are final weight and initial weight of the samplein grams.

Contact angle is measured with a VCA2500xe (VCA=Video Contact Angle)made by Advanced Surface Technologies (Billerica, Mass.). The fluid isdispensed by an automatic syringe that delivers the same amount eachtime. The camera takes the picture and the software measures the contactangle from the picture. The contact angle is reported in degrees.

EXAMPLES

Siloxyl polyether surfactant (Silwet, GE Silicones, Evansville, Ind.)was added to a spinning solution of DuPont Nylon 66-FE 3218 polymer informic acid. Webs were electroblown using the procedure given inpublication WO 03/080905 and produced the web properties listed in Table1 (individual examples+control).

TABLE 1 Surfactant Basis Mean Fiber Web Air Loading in Weight DiameterThickness Permeability Fibers (wt %) (g/m²) (nm) (mm) (cfm/ft²) 0.0 30.0360 0.152 6.02 0.42 29.5 446 0.162 5.52 0.83 28.9 453 0.162 5.56 1.2530.7 447 0.165 5.7

Table 2 shows the wetting behavior of the nanoweb samples containing 0wt %, 0.42 wt %, 0.83 wt % and 1.25 wt % Silwet, respectively. Thewetting speed was measured in water and 20% KOH. Two measurements weredone for each sample and both measurements are reported in Table 2,separated by commas. The results clearly show that the samples withsurfactant wet faster then the ones without any surfactant. In this case0.83 wt % or above of Silwet in the fibers was shown to be an effectiveamount.

Table 2 also shows the percentage electrolyte absorption of the nanowebsamples containing 0 wt %, 0.42 wt %, 0.83 wt %, and 1.25 wt % Silwet,respectively. The electrolyte absorption was measured in 35% KOH. Theelectrolyte absorption by nanowebs was significantly higher for sampleswith surfactant. In this case 0.83 wt % or above of Silwet in the fiberswas shown to be an effective amount.

TABLE 2 Surfactant Wetting Wetting Loading Electrolyte time of Time inFibers absorption Water 20% KOH (wt %) (%) (Seconds) (Seconds) 0.0 2744, 5 Not wetting 0.42 285 1, 1  7, 11 0.83 572 <1, <1 3, 4 1.25 458Instant <1, <1

Table 3 shows the contact angle of the nanoweb samples containing 0,0.42, 0.83, and 1.25 wt % Silwet, respectively. The contact angle wasmeasured with water and 20% KOH, respectively. Higher contact anglemeans poor wet-ability of nanowebs in that particular solvent. Sampleswith higher level of surfactants showed lower contact angles. Thewetting was very fast (“instant”) for samples with greater then 0.83 wt% surfactant and thus was not possible to get a read on contact angle.The data clearly shows that the wetting properties of samples withsurfactant (>0.83 wt %) was very good.

TABLE 3 Surfactant Contact Contact Angle Loading in Angle Water 20% KOHFibers (wt %) (degrees) (degrees) 0.0 124, 134 134, 138 0.42 Instant132, 134 0.83 Instant Instant 1.25 Instant Instant

1. An alkaline battery having a separator comprising a porous fine fiberlayer of wet-able polymeric fibers having a mean diameter in the rangefrom about 50 nm to about 3000 nm, wherein the porous fine fiber layerpermanently wets with strong alkaline electrolytes.
 2. The battery ofclaim 1 wherein the fiber polymer comprises an effective amount of asurfactant embedded therein.
 3. The battery of claim 1 wherein thefibers are produced from a spinning solution of polymer byelectrospinning or electroblowing and surfactant is added in thespinning solution.
 4. The battery of claim 2 wherein the surfactant ispresent at a level of about 0.4% to about 20% by weight of the polymer.5. The battery of claim 2 wherein the surfactant is present at a levelof about 1% to about 5% by weight of the polymer.
 6. The battery ofclaim 2 wherein the surfactant is a nonionic surfactant.
 7. The batteryof claim 1 wherein the fibers comprise a polymer selected from the groupconsisting of aliphatic polyamide, semi-aromatic polyamide, polyvinylalcohol, cellulose, polyethylene terephthalate, polypropyleneterephthalate, polybutylene terephthalate, polysulfone, polyvinylidenefluoride, polyvinylidene fluoride-hexafluoropropene, polyacrylonitrile,polypropylene, polyethylene, polymethyl methacrylate, polymethylpentane, polyphenylene sulfide, polyacetyl, polyurethane, aromaticpolyamide and blends, mixtures and copolymers thereof.
 8. The battery ofclaim 1 wherein the porous fine fiber layer has a mean flow pore size ofbetween about 0.01 μm and about 15 μm.
 9. The battery of claim 1 whereinthe porous fine fiber layer has a thickness of between about 0.1 mil(0.0025 mm) and about 12 mil (0.3 mm).
 10. The battery of claim 1wherein the porous fine fiber layer has a basis weight of between about1 g/m² and about 90 g/m².
 11. The battery of claim 1 wherein the fibershave a mean diameter between about 50 nm and about 1000 nm.
 12. Thebattery of claim 1 wherein the polymer is cross linked.
 13. The batteryof claim 1 wherein the separator comprises multiple porous fine fiberlayers.
 14. The battery of claim 1 wherein the separator comprisesmultiple porous fine fiber layers comprising differing polymers.
 15. Thebattery of claim 1 wherein the separator comprises multiple porous finefiber layers having differing characteristics selected from the listconsisting of thickness, basis weight, pore size, fiber size, porosity,air permeability, ionic resistance and tensile strength.
 16. Thealkaline battery of claim 1 wherein the alkaline battery is a Zn—MnO₂primary, Zn—MnO₂ secondary, Zn-Air, Zn—AgO, Ni—Zn, Cd—AgO, Zn—HgO,Cd—HgO Ni—Cd, Ni-Metal Hydride, or Ni—H₂ battery.
 17. The alkalinebattery of claim 2, wherein the polymeric fibers are further coated withsurfactant.